package test

[Jason Attin]

HI guys, I' ve picked up the book for the OCA exam, and I was just reading about packages. I tried the package access code sample, but, strangely, it compiles OK but it won't run. when I try

java packageb/ClassB 

I get:
Error:COuld not find or load main clas package.ClassB
So I have a temp folder C:\temp and C:\temp\packagea containing ClassA.java

package packagea;
public class ClassA{
}

and C:\temp\packageb containing ClassB.java

package packageb;
import packagea.ClassA;
public class ClassB{
	public static void main(String[] args){
		ClassA a;
		System.out.print("Got it");
	}
}

Any idea?
thanks

[Jesper de Jong]

The java command expects a fully-qualified class name and not a filename. (Unlike the compiler, javac, which expects a filename of a source file).

To run ClassB, use the command: java packageb.ClassB
(with a dot, not a slash).

There are a few things to remember with regard to packages:

1. The directory structure has to match the packages of your source files. It looks like you have that correct.

2. When you run a program with packages, then make sure you have the base directory of the package hierarchy in the classpath. If you don't specify the classpath explicitly (through the -cp or -classpath option or the CLASSPATH environment variable), then Java will take the current directory as the classpath, so then you should run your program from the base directory of the package hierarchy. In your example, you should be in the directory C:\temp on the command line.

So, this should work:

C:\temp> javac packagea\ClassA.java
C:\temp> javac packageb\ClassB.java

C:\temp> java packageb.ClassB

[Jason Attin]

thanks. Apologies, just realized that in fact I was using the right command

java packageb.ClassB

but didn't work.
Anyway, I was compiling both the classes in both the packages at the same time, but now instead I tried to compile them separately:

C:\temp> javac packagea\ClassA.java 
C:\temp> javac packageb\ClassB.java 

But when i compile ClassB I get this:

C:\temp>javac packageb\ClassB.java
packageb\ClassB.java:2: error: package packagea does not exist
import packagea.ClassA;
               ^
packageb\ClassB.java:5: error: cannot find symbol
                ClassA a;
                ^
  symbol:   class ClassA
  location: class ClassB
2 errors

C:\temp>

[Jesper de Jong]

That means that the current directory is not in the classpath. Did you set the CLASSPATH environment variable to anything? It's best to leave that variable unset.

If, for whatever reason, you do want to set it, then make sure that you add the current directory to it, which is indicated by a "." (point): CLASSPATH = C:\Blah\Whatever;.

Or, if you don't want to change CLASSPATH, then use the -cp or -classpath option when compiling and running your program:

C:\temp> javac -cp . packageA\ClassA.java
C:\temp> javac -cp . packageB\ClassB.java

C:\temp> java -cp . packageB.classB

[Julian West]

Hey, I have the same book.  I followed along and it worked fine.

Can you do a dir /s and copy/paste it here?

Before:

C:\SeaTemp>dir
 Volume in drive C is System
 Volume Serial Number is 5E40-0895

 Directory of C:\SeaTemp

2016-07-08  14:19    <DIR>          .
2016-07-08  14:19    <DIR>          ..
               0 File(s)              0 bytes
               2 Dir(s)  109,619,384,320 bytes free

C:\SeaTemp>md packagea

C:\SeaTemp>md packageb

C:\SeaTemp>cd packagea

C:\SeaTemp\packagea>copy con ClassA.java
package packagea;
public class ClassA {}
^Z
        1 file(s) copied.

C:\SeaTemp\packagea>cd ..

C:\SeaTemp>cd packageb

C:\SeaTemp\packageb>copy con ClassB.java
package packageb;
import packagea.ClassA;
public class ClassB {
 public static void main(String[] args) {
  ClassA a;
  System.out.println("Got it");
 }
}
^Z
        1 file(s) copied.

C:\SeaTemp\packageb>cd ..

C:\SeaTemp>javac packagea\ClassA.java packageb\ClassB.java

C:\SeaTemp>java packageb.ClassB
Got it

C:\SeaTemp>

After:

C:\SeaTemp>dir /s
 Volume in drive C is System
 Volume Serial Number is 5E40-0895

 Directory of C:\SeaTemp

2016-08-11  21:29    <DIR>          .
2016-08-11  21:29    <DIR>          ..
2016-08-11  21:35    <DIR>          packagea
2016-08-11  21:35    <DIR>          packageb
               0 File(s)              0 bytes

 Directory of C:\SeaTemp\packagea

2016-08-11  21:35    <DIR>          .
2016-08-11  21:35    <DIR>          ..
2016-08-11  21:35               195 ClassA.class
2016-08-11  21:29                43 ClassA.java
               2 File(s)            238 bytes

 Directory of C:\SeaTemp\packageb

2016-08-11  21:35    <DIR>          .
2016-08-11  21:35    <DIR>          ..
2016-08-11  21:35               421 ClassB.class
2016-08-11  21:31               163 ClassB.java
               2 File(s)            584 bytes

     Total Files Listed:
               4 File(s)            822 bytes
               8 Dir(s)  109,629,775,872 bytes free

C:\SeaTemp>

[Jason Attin]

Ah wait, the CLASSPATH variable, yes you're right sorry. Usually I include the path to the Java installation, and I forgot to do that here. Currently it is set to C:\Program Files (x86)\IBM\RationalSDLC\ClearQuest\cqjni.jar only, so I will add the path to java as I usually do. Let me try that first, failing that I will use

-cp

.

[Jason Attin]

OK, so changing the path variable doesn't work (although I changed to point to the directory where java is installed). So I tried this approach instead

C:\temp> javac -cp . packageA\ClassA.java
C:\temp> javac -cp . packageB\ClassB.java

C:\temp> java -cp . packageB.classB

which worked. Now, the problem is that this is different from what the OCA book says, so what do I do? which one should I follow?
thanks

[Dave Tolls]

Why have you set the CLASSPATH environment variable?

There's almost never a good reason to do this in any environment, especially not a dev one.

You also never need to add the Java installation directory in a classpath either.

[Jesper de Jong]

Note that the variables PATH and CLASSPATH are two totally different things.

The PATH is used by Windows to find executable programs to run. You should add the bin directory of the JDK to the PATH so that you can use the javac and java commands from the command prompt.

The CLASSPATH is used by Java to find *.class files. It's best to leave this not set at all, as already explained.

[Julian West]

Perhaps your CLASSPATH is set to something that doesn't include the current directory (.).

To check, at DOS prompt:
IF DEFINED CLASSPATH ECHO %CLASSPATH%

To add current directory to current CLASSPATH:
SET CLASSPATH=.;%CLASSPATH%

To clear it:
SET CLASSPATH=

cf. https://docs.oracle.com/javase/tutorial/essential/environment/paths.html

DOS command reference: http://ss64.com/nt/set.html